Refractory block furnace wall

ABSTRACT

A wall of a refractory furnace comprises a plurality of refractory blocks positioned in side-by-side and/or end-to-end relationship. Each of the blocks is provided with a through passage and a structural member traverses the passage for anchoring the blocks to a supporting structure. The structural member may pass through a number of blocks placed in end-to-end relationship and the fluid medium may be conveyed through the passages for thermally conditioning the wall of the furnace.

United States Patent Javaux [4 1 Apr. 11, 1972 REFRACTORY BLOCK FURNACE [56] References Cited WALL UNITED STATES PATENTS [721 lnvenm Gmve Java, Brussels Belgium 944,296 12/1909 Stimmel ..110/95 [73] Assignee: Glaverbel, Watermael-Boitsfort, Belgium 112491636 12/1917 Keyes "263/43 3,295,945 1/1967 Plumat ...65/182 X [22] F1led: Oct. 3, 1969 3,486,876 12/1968 Augustin et al. ..65/374 X [2]] Appl' 863617 Primary Examiner-Arthur D. Kellogg Attorney-Edmund M. Jaskiewicz Ii [30] Forelgn App cation Priority Data ABSTRACT Oct. 4, 1968 Luxembourg ..57030 A Wall of a refractory furnace comprises a plurality ofIefrac tory blocks positioned in side-by-side and/or end-to-end rela- [52] U.S. Cl. ..65/182 R, 65/346, 65/347, n-onship Each of the blocks is provided with a through 65/356' 263/461 263/48 266/43 passage and a structural member traverses the passage for [5 l Int. Clanchoring the blocks to a upporting struetura The tructural [58] Field Of Search ..65/374, 356, 347, 346, 182, member may pass through a number of blocks placed i endto-end relationship and the fluid medium may be conveyed through the passages for thermally conditioning the wall of the furnace.

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'INVENTOR GUSTAVE JAVAUX REFRACTORY BLOCK FURNACE WALL The present invention relates to a furnace having walls made of refractory blocks, more particularly, to the structure of such refractory blocks.

Considerable time is required for constructing a furnace wall from refractory blocks particularly when it is necessary to attach individually the blocks to a supporting structure which may comprise a frame work or metal walls. When it is intended that the furnace is to contain a bath of liquid material having a density greater than that of the refractory blocks each block must be firmly anchored to prevent detachment of the block from the supporting structure by buoyancy forces exerted upon the blocks by the liquid. These conditions exist when the furnaces are in the form of a tank or receptacle for containing molten metal.

It has been proposed to construct such refractory blocks with at least one recess within which an end of an attaching member is inserted and secured in position with refractory cement. The projecting ends of the attaching members are then connected to the furnace supporting structure such as by bolts or by welding. This proposed arrangement had the disadvantage that these cement bonds between the blocks and the attaching members were weak points which are particularly susceptible to deterioration because of the heat encountered under working conditions. In the event a block becomes detached the furnace supporting structure is exposed and is liable to serious damage if contacted by the molten metal bath in the furnace. This situation is particularly likely to arise when the furnace contains a bath of molten metal such as tin for the manufacture of flat glass by the float process.

The attaching of the refractory blocks in the construction of a refractory block furnace is only one aspect of the problem since thermal conditioning systems are generally required which include components mounted with in the furnace. These components generally pass through the furnace walls and great care must be taken to insure that the component is hermetically sealed to the wall. In the particular case of a float glass furnace the atmosphere of the tank must be carefully insulated from the outside atmosphere to prevent any oxidation of the metal bath which would adversely affect the quality of the surface of the formed glass ribbon.

It is therefore the principal object of the present invention to provide a novel and improved refractory block for use in the construction of a refractory furnace wall.

It is another object of the present invention to provide a refractory block which can be readily assembled to form a furnace wall and through which a fluid may be circulated for thermal conditioning of the furnace.

According to the present invention a refractory furnace wall may be constructed from a plurality of blocks of refractory material with each block having a through passage so that a fluid medium can be conveyed through the block. A structural member which may be tubular or have an angle configuration traverses the passage and may be used for anchoring the block to a supporting structure of the furnace. An assembly of such blocks may be mounted in end-to-end relationship with a single structural member passing through the aligned passages and then anchored to a suitable supporting structure. A number of such assemblies may be then positioned in side-byside relationship to form a bottom wall for the furnace. Each block may be molded onto the structural member passing therethrough or the passage may be formed so that there is a clearance between the passage wall and the structural member. A conduit may be positioned within a tubular structural member and fluid circulated through the conduit for thermal conditioning of the furnace wall. The conduit may be provided with a series of orifices along its length through which fluid may be discharged into the tubular structural member for thermal conditioning purposes. The spacing between successive orifices may decrease from the ends of the block toward the center thereof.

In a process for thermally conditioning a furnace wall constructed of refractory blocks according to the present invention a fluid is circulated through the passage or passages within the blocks at temperatures which are substantially different from the temperatures of the wall of the passage so that a substantially non-uniform thermal action is exerted along the length of the passage. A greater portion of the heat is transferred in the central area of the furnace than in the lateral areas thereof.

The process for thermally conditioning a furnace as disclosed herein is particularly applicable for a furnace for the production of flat glass wherein molten glass is floated and cooled on a liquid within a refractory furnace.

Other objects and advantages of the present invention will be apparent upon reference to the accompanying description when taken in conjunction with the following drawings, which are exemplary, wherein:

FIG. 1 is a vertical longitudinal sectional view through a float tank;

FIG. 2 is a vertical longitudinal sectional view in enlarged scale of a portion of the bottom wall of a flot tank and taken along the line IIII of FIG. 3a-b;

FIG. 3a-b is a vertical transverse sectional view in two parts of the same bottom wall and taken along the line III-III of FIG. 2;

FIG. 4a-b is a view similar to that of FIG. 3a-b of a modification of the furnace wall of the present invention;

FIGS. 5 and 6 are portions of FIG. 4a-b but in enlarged scale and showing different arrangements for attaching a refractory block wall to the metal bottom of a furnace; and

FIGS. 7-12 show various configurations of refractory blocks and cross sections of anchoring members according to the present invention.

Proceeding next to the drawings wherein like reference symbols indicate the same parts throughout the various views a specific embodiment and modifications of the present invention will be described in detail.

In FIG. 1 there is shown a simplified diagrammatic vertical longitudinal section through a float glass apparatus which essentially comprises a melting tank 1, a float tank 2 and an annealing lehr 3.

Float tank 2 comprises a bottom 4 and a crown 5 interconnected by side walls 6. There are also end walls 7 and 8 which are separated from the crown 5 by slots 9 and 10. These described components of the float tank are made of refractory materials. A metal wall 1 1 hermetically encloses the bottom 4, side wall 6 and end walls 7 and 8 of the tank within which is a bath of molten material 12.

The melting tank 1 contains a bath of glass 13 from which molten glass is cast over a casting lip 14 between casting rollers 15,16 which shape a glass ribbon 17. The glass ribbon 17 is then conveyed by a series of transporting rollers 18 to slot 9 of the float tank within which it is deposited on the bath of molten material 12 while continuing to move in the direction indicated by the arrow X. The glass ribbon is fire polished on the bath of molten material 12. The molten material 12 may comprise a molten salt but is preferably formed of a metal such as silver or tin. The glass ribbon 17 moves upon the bath 12 toward slot 10 and is conveyed by rollers 19 to the annealing lehr 3.

In FIGS. 2 and 3a-b there is illustrated a wall construction according to the present invention which can be used in constructing the float tank 2 of FIG. 1. Blocks 33 of a refractory material, such as carbon or silico-aluminous blocks, are arranged in rows of side-by-side assemblies over a metal wall 31 covered with a layer of cement 32 with the rows of assemblies extending transversely of the float tank. Each block is provided with a through passage 34 which forms a duct of rectangular cross section. The refractory blocks can be readily manufactured, such as by casting, with a passage or channel therein or the passage may be formed in the solid refractory block after its manufacture.

The end walls of each assembly of blocks contact metal side walls 35,36 each of which is covered with a layer of cement 37. A structural member 38 in the form of a rectangular tube extends through the passages of the aligned blocks with there being a small clearance around the tubular member. The tube 38 forms an anchoring member for the refractory blocks of that assembled row of blocks. The use of a single anchoring member common to all of the blocks of an assembly significantly simplifies the construction of a furnace wall. When two or more anchoring members traversing the blocks are employed a cross section of each anchoring member may be made smaller than when a single anchoring member is used. Accordingly, the passages extending through the blocks can therefore have a smaller cross sectional area with the result that there is less mechanical weakening of the blocks.

The ends of the anchoring tube 38 extend through openings 39,40 in the side walls 35,36 respectively and are supported outside the tank by fixing members indicated diagrammatically at 41 and 42 which permit axial elongation of the tube resulting from its thermal expansion. Refractory blocks 46,47 are positioned on the end blocks of each assembly against the corresponding side walls 35,36. The blocks 46,47 and the blocks 33 of an assembly thus form a tank for a bath of molten material 48 upon which a glass ribbon 49 floats and moves in the direction indicated by the arrow X.

Tungsten plates 50 having a density greater than the bath of molten material 48 are positioned on the blocks 33 of the assemblies forming the bottom wall of the tank. The plates 50 also have a higher thermal conductivity than the refractory blocks and thus promote thermal homogenization of the bath 48. Plates 50 also function as ballast for the assembly and protect the joints between the refractory blocks 33.

In some applications of the present invention a layer of thermally insulating blocks may be positioned between the refractory blocks held by the anchoring members and the exterior of the furnace. A combination of silico-clay (insulating) blocks and carbon (anchor) blocks is one example of the use of different kinds of blocks. The carbon blocks promote conducting the heat away from the molten metal in the furnace toward the exterior of the furnace through the intermediary of the fluid cooling medium flowing through the passage ways in the blocks whereas the insulating blocks permit high heat retention in the furnace at other times.

In constructing a furnace according to the present invention two or more block or block assemblies as described above may be positioned side-by-side to form the whole or a portion of a furnace wall and/or a number of such block or block assemblies can be arranged in a number of superimposed layers. When the furnace wall comprises two or more layers of blocks, the blocks traversed by anchoring members according to the present invention may be used for all or only one of the layers. When used in only one layer it is preferably that layer of blocks which is best able to stand up to the various forces, such as gravity and buoyancy of the molten liquid, which may be encountered in the furnace. For furnace walls having a relatively long span, at least two blocks or block assemblies may be positioned in end-to-end relationship and the corresponding anchoring elements being joined.

A fluid circulating conduit 43 is positioned within the tubular anchoring member 38. The conduit may be hair-pin shaped when the thermal conditioning must be differential. The conduit is provided with a plurality of orifices 44 from which fluid is discharged onto the inner surface of the tubular member 38. The orifices 44 are directed toward the upper portion of the tubular member which is the portion nearest to the molten bath 48. The orifices are more closely spaced, measured along the longitudinal axis of the tube, at the central portion of the assembly rather than at the ends thereof so as to reduce or control temperature gradients transversely to the bath 48. Thus more heat can be extracted at the center of the block assembly than at the ends. Heat insulating sleeves 45 may be positioned at the end portions of the conduit 43 and are axially displaceable thereon. Sleeves 45 control the amount of discharge of fluid at those locations where the sleeves are positioned and thus permit the required regulation of the transverse temperature gradient. The temperature gradient along the length of the tank can also be very effectively controlled by controlling the temperature and/or the flow of fluid in the conduits 43 passing through the assembled rows of refractory blocks. As a result, the length of the float tank may be reduced.

When a tubular anchoring member is used, the conditioning fluid may be circulated through the tube without contacting the refractory block or blocks. This is particularly desirable when a liquid conditioning medium is used which must not come into contact with refractory blocks. This feature is also advantageous because if a block becomes cracked while the furnace contains a bath of molten metal, the metal cannot enter the conditioning system.

Each row of refractory blocks may be assembled onto its anchoring member at the site where the float tank is to be installed. The refractory blocks 33 within which the passages 34 have already been formed are then placed in parallel rows on the tank bottom 31 which has been previously covered with the cement 32. The side walls 35,36 provided with openings 39,40 and with a coating of cement 37 are then placed against the end blocks of the assembled rows. The tubular anchoring member 38 passes end-wise through each opening 39 inside wall 35, through the passages 34 in the aligned blocks 33 and through the opening 40 in the other side wall 36 of the float tank. The attaching members 41,42 are then fitted onto the projecting portions of the anchoring tubes 38. Thermal conditioning conduits 43 and their heat insulating sleeves 45 can then be readily slid onto the tubular members 38. As the last step, the refractory blocks 46,47 are positioned on the end blocks of each assembly. The assemblies of such refractory blocks arranged side-by-side and extending transversely of the furnace thus build up the bottom and side walls of the tank.

The assemblies of blocks may also be formed prior to transporting the components to the installation site of the float tank. The refractory blocks 33 may be positioned in rows and the tubular anchoring members 38 then passed through the aligned passages of the blocks. As an alternative, the blocks 33 can be threaded onto the tubes which are to serve as the anchoring members. The present invention may also be ap plied by molding a single refractory block onto the tubular member 38. Such a construction is particularly useful for relatively short spans and where the refractory blocks and anchoring members will not undergo appreciable thermal expansion. Where the blocks are threaded onto the anchoring member there may be substantial clearance between the wall of the passages of the blocks and the anchoring member. Since it is not essential for the anchoring member to prevent entirely rotary movement of block or blocks, it is sufficient if the anchoring member prevents the blocks from being displaced from their intended positions in the overall structure. The blocks may be retained in position by supplementary means such as cement but the anchoring member or members eliminate the necessity for relying on any such supplementary means.

When the blocks are assembled in an assembly shop remote from the site of the float tank the combination of anchoring members and blocks may be stored to await transportation to the site where the assemblies will be used to form a tank wall as described above. The erection of a furnace employing the present invention is thus significantly simplified and accelerated. Further, the present invention eliminates relying upon a cement bond between an attaching member and a refractory block. Since it is not necessary to secure each block individually to a supporting structure of the furnace the construction of a furnace is significantly speeded up. In many cases the assembly of blocks can be fixed by the ends of the anchoring member to the supporting structure of the furnace.

Where a plurality of refractory blocks are anchored upon a single anchoring member it is not necessary that all the blocks of the assembly be identical or be connected to the anchoring member or members in the same manner. The end blocks of an assembly can be molded onto the anchoring member while intermediate blocks are merely threaded onto the member or interlocked with it as disclosed herein. Preferably, the anchoring member has a polygonal cross section which may or may not conform to the polygonal cross section of a block passage but functions to prevent the refractory blocks from rotating about the anchoring member.

In FIG. 40-!) there is illustrated another arrangement for supporting the ends of the anchoring member upon which a plurality of blocks 33 are threaded. The side wall of the tank 35 is not unitary with the metal bottom 31. A compensating unit 55 which may comprise resilient elements, such as springs or resilient washers, absorbs thrust on side walls 35 caused by the expansion of the refractory blocks. A washer 56 separates side wall 35 from the compensating unit 55. The anchoring tube 38 is supported by attaching member 57 which is integral with the metal bottom 31 of the float tank.

In the structure shown in FIG. 4a-b a thermal conditioning system for the bath 48 comprises tube 58 through which water is circulated and extending through the anchoring tubes 38. Sleeves 55 are displaceable axially over the tube 58 to permit controlling of the thermal gradient transversely of the bath 48. The sleeves 55 form a heat insulation which controls the transmission of heat between the water circulating in the tube 58 and the tin bath 48.

In FIG. 5 there is illustrated an arrangement for attaching an assembly of refractory blocks to the metal bottom 31 of the float tank. As described above each block or assembly of blocks may be attached to the supporting structure of the furnace at the ends of the anchoring members. Where the block assembly is of an appreciable length connecting members may extend between successive blocks and attach points of the anchoring member intermediate the ends of the assembly to the supporting structure. Only a few such connecting or attaching members are usually required, even with a relatively long span, and a single such connecting member in the central portion of the assembly will often be sufficient. In FIG. 5 the space between the ends of the two refractory blocks 33 is somewhat exaggerated in order to clarify the attaching structure. The block passage 34 and the anchoring tube 38 both have a circular cross section. Positioned between the two adjacent blocks is an attaching member comprising a collar 61 surrounding the anchoring tube 38 and having a stem 62 with a threaded end 63 which extends through a slot 64 in the metal bottom 31. On the threaded end 63 there is a nut 65 bearing against a washer 66. The slot 64 permits movement of the assembly perpendicular to itself in the longitudinal direction of the tank. The adjoining faces of the refractory blocks 33 are provided with recesses 67 to accommodate the attaching member described above.

In FIG. 6 there is illustrated an attaching member positioned between two adjacent refractory blocks 33 and comprising an attaching lug 70 welded at 71 to the lower portion of the anchoring tube 38. The attaching lug 70 has a foot 72 which is retained by two angle irons of appropriate shape welded to the tank bottom 31. The attaching system 71,72 also permits free displacement of the refractory block assembly perpendicularly to itself as with the structure illustrated in FIG. 5.

Where the refractory blocks are cast or molded directly onto the anchoring member one or more attaching members which may be in the form of a clamp or the like can be secured beforehand to the anchoring member so that in the final assembly the attaching members are embedded in one or more refractory blocks.

In FIGS. 7-12 there are illustrated several possible configurations of the sections of the passages 34 in the refractory blocks 33 and of the anchoring members. In FIG. 7 the cross section of the passage 34 and the anchoring tube 38 are circular and there is clearance provided between the anchoring tube and the wall of the passage. In FIG. 8, the cross sectional configurations of both the passage 34 and the anchoring tube 38 are square and in a similar manner clearance is provided between the tube and the wall of the passage.

In FIG. 9, the refractory block 33 is provided with a passage 34 which has a bend therein opening to a side face of the block. The bend is formed by a channel or branch 340 which communicates with passage 34 at an angle. With this inverted L-shaped channel arrangement each refractory block is fitted onto the square anchoring tube 38 by a vertical translation movement in which the tube enters channel 340 in the direction indicated by the arrow X. The tube 38 is then given a horizontal translation movement to move in the direction indicated by the arrow Y with respect to the block. This method of attaching a refractory block to an anchoring member is particularly advantageous when a block of the assembly must be replaced or the assembly is to be mounted without displacing the anchoring tube.

In FIG. 10, the refractory block is provided with three through passages 82-84 each having a smaller cross section than the passages in the previously described blocks. The passages 83,84 do not house anchoring members but are used for the circulation of a conditioning fluid, such as air. The passage 82 encloses an anchoring tube 38. In this structure the upper portion of the refractory blocks is conditioned more satisfactorily since the conditioning fluid is closer to the upper face of the block.

In FIG. 11, the through passage 34 has a rectangular cross section and is traversed by a cross-shaped anchoring member so as to form four ducts 81 through which a conditioning fluid can circulate in direct contact with the refractory blocks.

In FIG. 12, the anchoring member also comprises an angle iron but the angle iron is U-shaped as shown at 85 and forms with the wall of the passage a fluid circulating duct 86. Such a U-shaped angle iron will satisfactorily locate the refractory blocks having passages of a rectangular cross section if the base of the U contacts one wall of the passage while the parallel arms of the U are close to the adjacent parallel walls of the passage and thus prevent the blocks from rotating about the anchoring member.

A substantial advantage of the present invention is that thermal conditioning of the interior of the furnace can be carried out by conveying a suitable fluid through at least one passage way extending through a block or assembly of blocks. When the anchoring member through the block passage ways is tubular, it may function as a conduit to convey a thermal conditioning medium. This conduit need not extend along the full length of the block or block assembly. The thermal conditioning can be applied selectively or differentially along the block or block assembly in order to regulate the temperature in the float tank both longitudinally and transversely as may be desired. At least one conduit or passageway may be provided with differential conditioning elements which may comprise localized heat insulating units within the block passages and/or electrical resistances or cooling elements.

Thermal conditioning as employed in the present invention permits the temperature of the refractory blocks, and their resulting behavior, to be controlled. Such furnace chamber thermal conditioning elements are flexible in that they easily can be eliminated or reduced in number. This is of particular significance when the interior of the tank must be sealed from the outside atmosphere such as in the case of a float glass tank. In a float tank, the thermal conditioning system using passage ways in one or more assemblies of refractory blocks as described above can be employed instead of the usual cooling elements above the glass ribbon. This is an important advantage because where such upwardly located coolers are employed, cooling must be restricted in order to eliminate the occurrence of defects in the glass such as waves and drawn-out waves (discontinuous lines) which occasionally spoil the optical quality of longitudinally drawn glass. One effect of restricting cooling from the upper elements is a tendency for highly vitreous drops of glass, which have a composition very different from that of the float glass product, to form at the internal faces of the refractory blocks from vitreous phases of the blocks and then to rise and become drawn out in fine fibers between the refractory blocks and the float glass. The use of thermal conditioning elements in at least one passageway traversing the refractory blocks also eliminates the need for locating thermal conditioning elements in the metal bath. The location of elements within the bath is particularly disadvantageous since the tubes must be made of a special metal to resist corrosion and the depth of the bath must be considerably greater than otherwise required in order to accommodate the tubes. However, when thermal conditioning is achieved by conveying a cooling medium through a passageway in the refractory blocks the temperature of the metal bath can be readily regulated both longitudinally and transversely and the length of the tank can be decreased. The cooling can be regulated so that greater cooling takes place at the longitudinal center zone of the tank so that compensatory heating at the side or lateral areas to avoid excessive cooling of the sheet edges is avoided.

Although the present invention has been described with particular reference to float tanks it is to be understood that the invention also applies to other furnaces used either in the glassmaking industry or in foundry or metallurgical institutlons.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions.

What is claimed is:

1. In a refractory furnace for the treatment of glass floating on a tank containing a bath of molten material, a refractory furnace wall being formed of a plurality of refractory blocks positioned transversely within the said wall, each of said blocks having an aperture therein which is alignable with adjacent blocks as assembled to define a continuous passageway through the assembly of blocks, elongated block-anchoring means disposed within said passageway and extending throughout the length thereof and firmly anchoring the aligned blocks together, conduit means disposed within at least a portion of said anchoring means and having means for discharging a fluid into the interior of said block-anchoring means for thermal conditioning of said wall, and means for controlling the zone of discharge of said fluid for regulating the temperature along the length of said passageway.

2. In a refractory furnace wall as claimed in claim 1 wherein said anchoring means is tubular.

3. ln a refractory furnace wall as claimed in claim I wherein said anchoring means is an angle.

4. In a refractory furnace wall as claimed in claim 1 wherein each block is molded onto said anchoring means.

5. In a refractory furnace wall as claimed in claim 1 wherein said anchoring means has clearance within said passage.

6. In a refractory furnace wall as claimed in claim 1 wherein said conduit means has fluid discharge orifices within each block.

7. In a refractory furnace wall as claimed in claim 6 wherein there being a series of said orifices along the length of said conduit means with the spacing between successive orifices decreasing from the ends of the block toward the center thereof.

8. In a refractory furnace wall as claimed in claim 6 and comprising means upon said conduit and displaceable longitudinally thereon for masking said orifices to control the discharge of fluid from the conduit.

9. In a refractory furnace wall as claimed in claim 1 and means for transferring a greater portion of the heat in the central area of the furnace than in the lateral areas thereof. 

1. In a refractory furnace for the treatment of glass floating on a tank containing a bath of molten material, a refractory furnace wall being formed of a plurality of refractory blocks positioned transversely within the said wall, each of said blocks having an aperture therein which is alignable with adjacent blocks as assembled to define a continuous passageway through the assembly of blocks, elongated block-anchoring means disposed within said passageway and extending throughout the length thereof and firmly anchoring the aligned blocks together, conduit means disposed within at least a portion of said anchoring means and having means for discharging a fluid into the interior of said block-anchoring means for thermal conditioning of said wall, and means for controlling the zone of discharge of said fluid for regulating the temperature along the length of said passageway.
 2. In a refractory furnace wall as claimed in claim 1 wherein said anchoring means is tubular.
 3. In a refractory furnace wall as claimed in claim 1 wherein said anchoring means is an angle.
 4. In a refractory furnace wall as claimed in claim 1 wherein each block is molded onto said anchoring means.
 5. In a refractory furnace wall as claimed in claim 1 wherein said anchoring means has clearance within said passage.
 6. In a refractory furnace wall as claimed in claim 1 wherein said conduit means has fluid discharge orifices within each block.
 7. In a refractory furnace wall as claimed in claim 6 wherein there being a series of said orifices along the length of said conduit means with the spacing between successive orifices decreasing from the ends of the block toward the center thereof.
 8. In a refractory furnace wall as claimed in claim 6 and comprising means upon said conduit and displaceable longitudinally thereon for masking said orifices to control the discharge of fluid from the conduit.
 9. In a refractory furnace wall as claimed in claim 1 and means for transferring a greater portion of the heat in the central area of the furnace than in the lateral areas thereof. 